Recent advances in mobile computing and energy-efficient communication have shown promise in the continuous acquisition, storage, and processing of physiological signals. Pervasive sensors deployed in next generation networks have enabled algorithms capable of efficient and accurate information processing. The monitoring of physiology of a person including, but not limited to electrocardiogram (ECG or EKG) signals and Electroencephalography (EEG) signals may be performed with electronic devices which are used very regularly by any person.
The monitoring of the ECG signals are performed by interpretation of the electrical activity of a heart over a period of time, as detected by electrodes attached to surface of the skin and recorded by a device external to a body of a person. The recording produced by electrical activity of heart is termed as an ECG. The monitoring of EEG signals is performed by recording and interpretation of electrical activity along the scalp of a person. The EEG measures voltage fluctuations resulting from ionic current flows within the neurons of the brain.
Known in the art are methods for measuring physiological signals of a person such as breathing, body temperature, oxygen saturation and blood pressure. For monitoring breathing of a person, a spirometer may be used. A spirometer is an instrument that measures air flow while breathing and estimates the air capacity of the lungs, for diagnosing asthma or Chronic Obstructive Pulmonary Disease (COPD). The body temperature is an important indication of health including fever, sepsis, heat rash, or any other disease which may affect the persons. Measurement of temperature may be carried out using an electronic device with an Infra Red (IR) sensor.
Pulse oximeters may be used to monitor Oxygen saturation in a person, which is measurement of concentration of oxygen in arterial blood reaching tissues. The peripheral oxygen satuaration (SpO2) can be measured non-invasively by using a pulse oximeter. A pulse oximeter device comprises a photodetector that responds to red and infra-red light through tissue, such as finger tip, ear lobe, etc. and then processes the signal to estimate SpO2. Arterial Blood Pressure (BP) is another physiological signal that can be estimated non-invasively, using many techniques including the oscillometric principle. The amplitude of pressure change in a cuff on the upper arm of a person, which is inflated and then deflated, is sensed by a pressure sensor. The sensed signal is processed to estimate systolic and dialostic BP. For measuring any physiological parameter, plurality of sensors are used to sense some specific aspect of underlying physiology. The sensed signals are then processed using software algorithms running on special purpose medical devices, general purpose computers, micrprocessors, ASICs, or any other computing device.
Recently, cellphones, smartphones, tablets and other personal devices have become ubiquitous. These devices along with laptops and desktop computers are herein after referred as a Host device. Most of the host devices provide a headphones socket that provides audio signal to the ear buds. Also, they provide power to microphone and receive the signal from the microphone. Headphones that incorporate microphones for telecommunications are typically called headsets. Hereinafter in this description, the terms headphone and headset are used interchangeably. A headphone jack and a headphone socket require a physical, electrical connection to interoperate. Hereinafter in this description, the terms jack and socket are used to represent any electrical connection. The wiring for headphones and headsets using typical 3.5 mm, 4 pin jacks and compatible sockets is known as TRRS (tip, ring, ring, and sleeve) configuration as shown below.
Pin NumberPin NameDescription1TipLeft Audio Out2Ring 1Right Audio Out3Ring 2Common/Ground4SleeveMicrophone Input
Known in the art is a headphone or electret microphones cable with a headphone jack on one side, which may be interfaced with the Host device. The headphones commonly used in hands free, voice call applications require microphone-bias to power a preamplifier that is internal to the microphone assembly. The socket on a host device or mobile device or consumer device provides the bias on the Microphone input (pin-4) and Common/Ground (pin-3), while receiving the audio signal from the microphone on the same two pins. The essential point is that power to a sensor and signal communication from the sensor is accomplished on a two-wire interface. A limitation of this AC-coupled architecture is that the input signals can only be AC signals, without having any information in the lower frequencies of the input signals such as audio and speech.
FIG. 1 shows a typical microphone connection to headphone socket on Smartphones and Tablets. The junction gate field-effect transistor (JFET) provides amplification and impedance matching. The resistor R values are in the rage of 1-10 KΩ which provides the required bias voltage and sources current for JFET operation. Also, the capacitor C values are in the range of 1-50 μF and blocks DC voltage while passing audio signals for ADC. The microphone becomes a current source, delivering few hundred μA. Thus, the two-wire interface can provide power to the sensor and also receive the analog data from the sensor using ac-coupled interface.
FIG. 2 shows a conventional ECG Equivalent Circuit. The impedance of the circuit of FIG. 2 for the commonly used electrodes is shown in FIG. 3. The impedance can be sometimes as high as 500 kΩ. This variability is due to the variation in off the shelf Ag/AgCl electrodes and due to aging in disposable electrodes. The ECG signals have a bandwidth in the range of 0.05 to 100 Hz. Typical amplitude of ECG signals is about 5 mV. The signals can sometimes ride on a DC bias of ±300 mV. There is significant diagnostic information in the lower frequencies that would be filtered by the high pass filtering action at the audio socket in smartphones. The reusable sintered Ag/AgCl electrodes mitigate the electrode variability to a large extent. Similarly, for signals such as air flow, temperature, light intensity, and pressure which correspond to certain physiological aspects of interest to clinicians, the frequency content is well below that of speech and audio. As the capacitor C in FIG. 1, will block low frequency signals, such a socket is not useful for receiving physiological signals.
In a patent application US 20130331663 discloses heart monitoring system usable with a smartphone or computer. It discloses a personal monitoring system with a sensor assembly to sense physiological signals. The system requires frequency modulated (FM) physiological audio signal and requires a carrier frequency to be in the range of 6-20 kHz. Here, the FM signal is an audio signal and is not an electric signal. Also, the audio transmitter of the personal monitoring system transmits an audio signal to the microphone.
A U.S. Pat. No. 8,509,882 discloses a personal monitoring device usable with a smartphone or computer. The device uses a frequency modulation (FM) demodulation and generates acoustic signal. The carrier signal is in the range of 6-20 kHz. The device uses an in-built microphone and an audio isolation transformer to interface to smartphone or a computer.
Typically, for headphones, powering the microphone sensor using microphone bias provided by the electronic device is known. The microphone sensor is limited to sensing speech, audio and ambient sound signals. The audio jack interface of the electronic device is not used for ECG, EEG and other physiological signals. Systems for powering devices connected to the audio jack and provide digital communication with the phone are known in the art. In these systems, the phone generates an audio signal that is rectified and filtered to generate power for an external device. The microphone port is reserved for communicating discrete data from the device to the phone. The microphone bias is not used to power external devices.
Accordingly, a need exists for a device and method for monitoring physiological signals using cellphones, smartphones, tablets and other personal devices.